The present invention relates to diffractive optical encoders, and more specifically to optical encoders employing multiple tracks such as a position sensing track and a separate index mark track.
Diffractive optical encoders are well known in the field of position sensing systems. A recent trend has been to develop diffraction based encoders of reduced size. Generally, such reduced size encoders are characterized by their use of a solid-state source of quasi-monochromatic (or nearly monochromatic) illumination, one or more binary gratings on a scale, one or more detecting elements, and a reduced number of additional optical components.
Recently developed encoders have incorporated vertical cavity surface emitting laser (VCSEL) light sources because of their convenient optical properties. Unlike the more traditional diode lasers, VCSELs emit light from a circular aperture on the top surface. This can provide for a nearly circular beam that can be easily pointed toward the scale without the need for additional optical elements. When included in a so-called Talbot encoder, the VCSEL helps create a small, low power, and easy-to-assemble package.
It has been known to employ multiple grating tracks on the scale of an optical encoder. For example, it has been known to include an index, or reference, mark track on the scale in addition to a position scale. An absolute diffractive encoder that uses three or more grating tracks on a single scale is also known.
While it is desirable to use a VCSEL in multi-track encoders, the applicants have discovered that the relatively small angle of the cone of light emitted by available VCSELs can present problems. Simply stated, over the dimensions of interest the VCSEL beam does not spread out enough to cover all the tracks. In some encoders this problem has been solved by increasing the distance between the VCSEL and the scale. However, this solution runs contrary to the goal of creating a compact encoder system.
In addition, when the VCSEL's beam is spread out over a large enough circular region to cover all tracks, only a small fraction of the VCSEL's light is actually used by the encoder. The maximum light power for illuminating the main signal track is effectively reduced by a factor equal to. the number of tracks. Additional power is lost in light directed into non-functional areas of the scale and the detector, for example into the gap between scale tracks. These factors may reduce the light power for the main signal track to less than half that of comparable encoders employing edge-emitting lasers, resulting in a correspondingly lower signal-to-noise ratio. Furthermore, unlike edge emitting lasers, VCSELs have inherent power limitations that prevent compensating for the reduced light power by driving the VCSEL with higher current.
Thus, there is a need for an improved method of illuminating a multi-track diffractive scale without sacrificing the benefits of small size generally associated with VCSEL-based encoders.